1. Technical Field of the Invention
The invention belongs to the field of remote measurement of physical quantities, in particular through the use of a radiofrequency link and a passive component. More specifically, the invention relates to resonant structures using bulk waves within piezoelectric films. The structure that is in accordance with the invention is optimized in order to increase its quality factor and coupling coefficient in its frequency range.
The invention also relates to a process for manufacturing such a hybrid structure, and to various uses.
2. Description of Background and/or Related and/or Prior Art
In the area of transponders and/or sensors, narrow band resonant devices are increasingly used, in particular those which operate in the 500 MHz range.
In particular, since the 1920s, resonators based on the vibrations produced in strips of piezoelectric materials have been produced, such as schematically illustrated in FIG. 1, with this type of component 1 being made up of two facing electrodes 2, 3 gripping a plate of piezoelectric material 4. When a radiofrequency field RF is applied to the terminals of the dipole 2, 3 formed in this way, a reverse piezoelectric effect results in the deformation of the plate 4 in accordance with the couplings which are allowed by the crystalline orientation of its component material. A resonance phenomenon is produced when the RF excitation frequency F corresponds to the phase velocity V for the movement that is generated divided by twice the thickness e of the plate (F=V/2e): the displacements at the surface of the plate 4 are then of maximum amplitude, whilst the stresses C reach their maximum values at its centre. Quartz has proven to be the most favored material for this type of application as a result of its thermo-elastic properties (high mechanical quality coefficients, the existence of orientations which compensate for the effects of temperature, etc. . . . ).
However, in order to achieve increases in frequency, it becomes necessary to reduce the thickness of usually massive materials to thicknesses e which introduce an element of risk into any industrial application (where the minimum thickness of plates is of the order of 30 μm): for a resonator 1 operating at its fundamental mode at 300 MHz by means of a shear wave at 3500 m·s−1, the thickness e of the plate 4 of an quartz AT cut should be of the order of 6 μm. Even using higher harmonics and other quartz cuts (such as BT cuts which allow the use of a wave which is propagated at 5100 m·s−1), a frequency of 1 GHz represents the practical limit for the operation of classical bulk wave resonators.
These devices have therefore been supplanted for radiofrequency signal processing applications by passive surface wave components (SAW: “Surface Acoustic Waves”) which have shown themselves to be capable of reaching frequency ranges which exceed gigahertz, and have seen a number of applications, such as remote querying of passive sensors. Such surface wave resonant structures, however, involve certain size constraints associated with the acoustic wavelength and even with their configuration, which needs to be of a minimum length in order to carry out their spectral function.
Furthermore, production of thin piezoelectric layers on non-piezoelectric substrates has been developed and it has been shown to be possible to excite bulk waves in piezoelectric films, often with longitudinal polarization and which simultaneously exhibit very high propagation speeds and high levels of piezoelectric coupling (several %). Different thin piezoelectric film bulk resonators 5 have been perfected: Resonators with thin films 6 on a substrate 7 (TFR: “Thin film resonators”), surface machined or bulk machined (FBAR: “Film Bulk Acoustic Resonator”, HBAR: “Harmonic Bulk Acoustic Resonator”), are illustrated respectively in FIGS. 2A and 2B, and the Bragg mirror 9 resonator 8 (SMR: “Solidly Mounted Resonator”) is shown in FIG. 2C.
The main technological difficulty in manufacturing such a component rests in the elimination, or local reduction in the thickness of the substrate 7 found on the rear face of the layer 5 in order to allow vibrations to be freely established: the substrate 7 which remains beneath the piezoelectric membrane 5 seems likely to generate interference modes. A composite solution was therefore envisaged allowing coupling of the advantages associated with the use of a thin piezoelectric layer 5 whose character differs from that of the substrate 7, based on the operating principles of bulk wave delay lines.
The simplest delay lines are made up of a piezoelectric transducer/propagation material/piezoelectric transducer composite structure in which bulk waves W are propagated: the delay depends on the length of the acoustic path that is traveled. A simplified version of this structure (FIG. 2D) combines the input and output transducers and the acoustic path involves reflection on a flat surface. This last structure 6′, the so-called composite resonant structure, exhibits a multiplicity of modes which correspond to the possible harmonics of the fundamental mode of structure 6′, whose frequency is always given by the relationship F=V/2e where e is the effective thickness of the thin layer 5/substrate 7′ composite plate and where V is the equivalent velocity for the mode, which principally depends on the elastic properties of the substrate 7′ (preferably a single crystal), slightly disturbed by the film 5; the polarization is set by the piezoelectric couplings of the film 5. The choice of the thicknesses of the various layers, and in particular of the single-crystalline strip 7′ is made so that one of the harmonic resonances of N order for the stack corresponds to the desired operating frequency F0 of the resonator 6′; furthermore, the dimensions given to the structure allows a spectral gap to be obtained between two resonances very much higher than the working frequency band (for example 1.8 MHz for the band centered on 433.92 MHz). Thus the chosen resonance may be selected with precision.
For a composite structure 6′ as shown in FIG. 2D, it would seem necessary to obtain an optimum coupling coefficient in order to limit insertion losses. In cases of a layer 5 of AlN deposited on a substrate of silicon 7′, even if it is possible to optimize the coupling coefficient value KS2 by varying the thickness of the substrate 7′, obtaining a value of 1‰, the Rayleigh wave coefficient on the quartz which is the lower limit for effective operation of the resonator 6′, is subject to the use of the fundamental mode. Such a small coupling coefficient is just compatible with most applications envisaged for remote measurement.
It has certainly been proposed (Shih-Yung Pao et al.: “Analysis and experiment of HBAR frequency spectra and application to characterize the piezoelectric thin film and to HBAR design”, Proceedings of the IEEE International Frequency Control Symposium 2002; 8A-5) that the thickness of the upper electrode 2 be increased in order to remedy this problem; this optimization of the coupling factor KS2 is carried out to the detriment of the over-voltage coefficient value for the resonant structure 6′ however, and therefore affects its possible use.